Process for the recovery of ammonium thiosulphate and thiocyanate



Oct. 16, 1945.

J. A. SHAW PROCESS FOR THE RECOVERYQF AMONIUM THIOSULEHATE AND THIOCYANATE Fil ed Oct. 11, 1944 2 Sheets-Sheet 1 225 uucuopm INVENTOR. Jo EPH 4. Sum.

J. A. SHAW 2,386,985 PROCESS FOR THE RECOVERY OF AMONIUM THIOSULPHATE AND THIOCYANATE Oct. 16, 1945;

Filed Oct. 11, 1944 2 Sheets-Sheet 2- Joroudqmb ukgc MPEG1 UB5 az 0559300 dokquqn Him INVENTOR. Jose/w 4. 5mm: BY g V nrrolvar.

Patented Oct. 16,.

UNl'TEQgSTATES PATE T oF i-ca 2,386,985 rnocs'ss non THE ancovnmz 0F AMMONI- A v UM 'rnrosvprna'rn AND THIOCYANATE Application October 11, 1944, Serial No. 558,262

' both salts can 9 Claims. (on. 23-75) The present invention relates in general to the separation and recovery of water-soluble thiocyanates and thiosulphates from aqueous solutions thereof and has reference more particularly to the recovery of substantially pure ammonium thiosul- Phate and ammonium thiocyanate from gas liqthiocyanaf How ver. if the discovered lower \iors containing the same. The invention is paraqueous p e. which ins themaior portion ticularly us'ei'ul in the separation of water-soluble of the t l lph tab removed from contact with thiocyanates and thiosulphate occurring in th the upper alcohohc phase and be thereafter divers liquors o1 aby-product coke plant, treated with additional quantities of alcohol to It is well known that certain thiocyanates are r itate ammonium hl s phatc. the B soluble in alcohol and in water, and that thiosuL- tamed thlosulphate when d h $100110! phates are insolublein alcohol. Upon this fact a dried is substantially e of tlflwyanatethere have been based methods for the separation In the accompanying drawings forming a part of the two said salts especially with th purpose of this spec fication and showing for purposes oi of producing pure thio n t o method, I exemplification a preferred apparatus and procfor example, comprises the evaporation to dryness 95s in Whlch the invent")? may be embmugd and of the salts, and subsequent extraction of the practiced but Withwt lilmtmg the f inventhiocyanate with alcohol. In this method, a ma- 111011 specifically t $11011 l s at ve stance or inlor portion of the thiosulphate is decomposed by stances:

the heat employed in evaporation oi the salt solution.

It is also well known that the addition of a large quantity oi alcohol to a solution containing ammonium thiocyanate and thiosulphate will precipitate out the ammonium thiosulphate and thus it can be obtained without decomposition;

but the process is commercially infeasible because j whenever the concentration of thiocyanate in solution is at all substantial the ammonium thiosulphate so obtained is contaminated with thiocyanate which is difllcult, if not impossible, to remove even by repeated alcohol washings.

The primary object or the present invention is the provision of an improved process and means for recovering substantially pure ammonium tliiosulphate salt from an aqueous solution containing ammonium thiosulphate and ammonium thiocysnate.

Another object of the invention is the provision of an improved process for theseparation'of ammonium thiosulphates from thiocyanates whereby the processes or apparatus hereinafter described or claimed.

It ha now been discovered that, by adding 8.1-

cohol slowly to an aqueous solution of ammonium thiosulphate and thiocyanate, the solution can, before thiosulphate salt is precipitated therefrom,

be separated into two liquid phases, a predominantly alcoholic upper phase and a predomibe more easily and economically nantly aqueous lower phase. It a further quantity or alcohol is added to the solution, the said aqueous phase disappears and thiosulphate salt precipitates. The so-separated salt is, as before described, contaminated with difflcultly removable ess can be conducted continuously.

Referring to the drawings and particularly to Fig. l, a phase separator 2, that can be simply a tall tower operating as a settling tank, is approm imately half-filled with a mixed aqueous salt solution containing ammonium thiocyanate and mmsulphate, which is flowed to thesaid separator through valved line 4. Alcohol, containing some water in solution and also some salt is thereafter pumped from a dilute alcohol storage and pumping tank 8 by pump 6 through a pipe to and is flowed gradually therethrough into the said phase separator 2.

The so-added alcohol forms a solution with the aqueous salt solution until there has been added a volume of alcohol approximately equal to the volume of the said aqueous salt solution, depend ing however upon th relative salt concentration in solution. Atabout thi said ratio of added alcohol to original aqueous solution, a, predominantly aqueous salt solution appears atthe base of the separator. After sumcient alcohol has been fiowed into the separator to form the two said liquid phases and before any solid phase has been tormed, the addition of alcohol is stopped and the two phases are permitted to settle and to separate.

The separated, lower, predominantly aqueous phase is drained from the said phase separator 2 through pipe I2 into a tank It wherein the ammonium thiosulphate of the said aqueous phase is precipitated out by the addition thereto of further quantities of alcohol, this alcohol being a wash alcohol derived from another step of the pipe l8 and into a hopper-shaped separatingtank 20, and is allowed to settle therein. The liquid so separated from solids of the slurry is decanted from the said separator 20 through a, pipe 22 into the pumping tank 8. The settled slurry is drained from the separating tank 20 into a centrifuge 24, wherein the precipitated salt is substantially dehydrated and is washed with alcohol for removal of substantially all traces of thiocyanates. The dried, washed precipitate constitutes substantially pure ammonium thiosulphate. After centrifugalization and washing, the thiosulphate is delivered from the said centrliuge 24 to a storage hopper 26. A valved pipe 21' connecting the salt separator 20 to pipe 22 can be employed to effect complete drainage of the said separator upon completion of treatment of the said aqueous phase.

After the predominantly aqueous phase has been substantially completely drained from the said phase separator 2, the supernatant, predominantly alcoholic phase is drained through the said valved line l2 into the tank It wherein this phase also is treated with another addition of alcohol either from storage or the above described washing step, and substantially all of the thiosulphate remaining in the alcoholic phase is precipitated therefrom. The slurry so produced is pumped by pump [6 to the salt separator 20 diluted with water that its efllcacy in the phase separating step will be impaired. In such an instance, the dilute alcohol can be pumped from the pumping'tank 8 through a pipe 50 having a valve 52 and through which it can be delivered to customary still means for rectification and subsequent return to the alcohol storage tank 32. A pipe 54 connecting the pipe 34 directly to the said alcohol storage and pumping tank 8 and having a valve 56 can, by opening the valve, be used to conduct non-diluted alcohol directly to the phase separating step. Thus, the amount of water returned to the said phase separator 2 can be regulated within those limits making feasible the use of convenient proportions and the obtaining of optimum yields of thiosulphate in the said phase separating step.

In accordance with the present invention, the apparatus may be so constituted, and modifications in process are so provided, as to achievea continuous process. Such provision, as shown more particularly in Fig. 2, includes a continuous phase separator 60. This separator may be one of any of several designs and will contain, for example, means 62 for agitating and securing intimate mixing of incoming mixed-salt solution and alcohol, and a settling chamber 64 for securing the separation of the two liquid phases formed 7 by the incoming liquids.

and the salt content of the slurry is settled out.

This salt, which consists of impure thiosulphate. is returned to the salt solution entering the phase separator 2.

The alcoholic solution from which thiosulphate has been separated is flowed from the separator 20 through pipes 22 and 28 into the collecting tank 30. Usually substantially pure ammonium thiocyanate can be recovered from the alcoholic solution by simple evaporation and crystallization steps and the separated alcohol can either be returned directly to the process or, if necessary,

be rectified in the customary manner and the product employed to replenish the alcohol in a storage tank 32. If an appreciable amount of ammonium thiosulphate remains in the dilute alcoholic solution, this contaminant can be destroyed by treatment with a mineral acid and the solution can be filtered before the said evaporation and crystallization steps are employed.

Alcohol from the storage tank 32 is drained therefrom through header 34 and valve 38 for various points in the process. Alcohol for washing thiosulphate crystals is directed into centrifuge 24 by opening a valve 38 in a, pipe 40 extending from the said header 34, and, for precipitating thiosulphate can be delivered directly to the salt precipitator l4 by opening valve 42 in a pipe 44, also branching from the said header.

Wash alcohol from the centrifuge 24 is drained by opening valve E6 in a pipe 48 through which such alcohol also can be flowed to the said salt precipitator. This supply of alcohol for the saltprecipitating step is usually the only alcohol added to the salt precipitator, since enough alcohol is ordinarily employed in washing the thiosulphate crystals to precipitate a later batch of salt.

' The recirculated alcohol in the described salt recovery system will in some instances become so Aqueous solution of thiocyanate and thiosulphate salt is pumped continuously into the inner, mixing cylinder of the separator 60 at a measured and controlled rate from a storage tank 68 therefor, by a. pump 68 and through pipe I0, having a rotameter 12 disposed therein. Within the phase separator 80 aqueous solution and alcohol added in predetermined proportion to the said solution in a manner as hereinafter described, mix thoroughly, form two phases, and thereafter separate, in the settling chamber 64, into a supernantant predominantly alcoholic solution and a lower predominantly aqueous solution containing substantially all the thiosulphate salt.

The lower aqueous phase is continuously drained from the base of the said phase separator 60 and is flowed through pipe I4 to a salt precipitator 18 wherein an additional stream of alcohol is added to the inflowing aqueous phase. A slurry is formed by such alcohol addition and is pumped from the salt precipitator 18 by a pump 80 through pipe 82 to a. salt separator 84 similar to the salt separator 20 previously described.

Settled slurry from the said salt separator 84 is flowed into a continuous centrifuge 86- such, for example, .as the type invented by Dr. Gustav ter Moor and manufactured by the Baker Perkins Company. Therein the thiosulphate of the aqueous slurry is substantially dehydrated, is washed by alcohol flowing from an alcohol storage tank 88 through pipe 90, and is discharged to a product storage hopper 82, all by completely automatic, continuous operation. Wash alcohol flows from the said centrifuge through pipe 94 to the said salt precipitator 18 wherein it is used to precipitate the thiosulphate salt and wherein it. becomes a part of the slurry of precipitated salt and alcohol thereto through pipe I04.

Alcohol can be delivered to the said pumping tank 08 directly from the storage tank "through.

a pipe I08, so that predetermined proportions of redistilled alcohol and dilute alcohol which has beenemployed to precipitate and to wash thio- ,sulphate salt, can be continuously mixed. The

' By controlling the proportion of dilute and redistilled alcoholin the aforesaid step, the mixture can be maintained at a concentration making continuously separable two liquid phases upon its admixture at controlled rate with aqueous salt solution.

The supernatant predominantly alcoholic phase is continuously decanted from the top of the phase separator 80 and is flowed through a pipe I I d to a salt precipitator I I6. Alcohol which is pumped from the alcohol storage tank 88 by a pump II8 through a pipe I20 is continuously added to the infiowing alcoholic phase in the preclpitator IIB. A slurry of thiosulphate precipitate is formed by such alcohol addition and is pumped from the salt precipitator I It by a pump I22 through a pipe I24 to a salt separator I26 similar to the salt separator 84 previously described. The impure thiosulphate salt which is obtained from the said salt separator I26 is returned to the said storage tank 68 for.

reprocessing;

Continuous overflow of alcohol solution from the salt separator I26 flows through a'pipe I28 to the thiocyanate-solution storage tank I30 whence it can be flowed to distillation means for rectification of the alcohol therein and to evaporation and crystallization means for recovery of the ammonium thiocyanate contained therein.

Example No. 1

v ammonia sulphur-arsenic compound, thereby driving of! ammonia and precipitating arsenic compounds which were thereafter filtered from the'said solution. Analysis of the so-evaporated solution was thefollowing: I

Specific gravity 1.258 at 25/25 Ammonium thiosulphate 373grams per liter Ammonium thiocyanate 454 grams per liter Boiling point (atmJ 117 C.

. i 3 twice its volume of alcohol. Aslurry was formed by the said addition of alcohol, the solid phase consisting of substantially all the ammoniumthlosulphate of the said lower phase, and a liquid phase consisting of an alcohol-water solution containing slight quantities of ammonium thiocyanate and thiosulphate. The 'so-form'ed slurry was centrifuged and the dried salt was washed with alcohol, whereby a white, high quality ammonium thiosulphate was obtained.

Example N0. 2

In a second exampleof batch operation, 77% of the ammonium thiosulphate of the treated concentrated solution was contained in the lower liquid phase which had a volume of less than one-quarter of the total liquid volume in the 1 separation step. Since the evaporated Thyl'ox solutions exhibit varying ratios of thiosulphate' to thiocyanate, it is therefore important to efll-.

the ratio of thiosulphate to thiocyanate varies as much as from 0.8 to 1.5, if the specific gravity of the mixed salt solution is maintained below about 1.25, the formation of a solid phase during the first separationcan be avoided. finding wasmade on the basis of runs made at 25 C. and when employing equal volumes of 95% alcohol and solution. It was further found that increasing the temperature of the liquids, and especially that returning some water in'soluti'on in the alcohol in the continuous method, would aid in avoiding formation of solidphase. ;Additional provisions, as have been heretofore described and which include means for controlling the rate of flow of mixed salt'soiution and of al-- cohol to the phase separator, and the concentration of-water in the alcohol to be flowed to the' phase separator, may be made for the continuous operation of the present process and avoid the formation of solid phase in the first step, and the An'equal volmne of 95% alcohol wasladdedto the above solution, whereupon there was formed two liquid phases, an upper, predominantly 'alj coholic phase and a lower, predominantly aqueous phase, in the ratio by volume, of'respectively 3.71 to 1. The phases analyzed as follows:

- Approximately 67% of the ammonium thiosulphate was absorbed in.- the lower layer which was withdrawn from contact with the said upper phase and was thereafter treated with approximately consequent contamination of thiosulphaie salt.

Example No.3 I A solution containing 302 grams per liter ammonium thiosulphate and 358 grams per liter ammonium .thiocyanate was heated with one and one-half volumes of alcohol at 30 C. A lower concentrated aqueous layer of ammonium thiosulphate'was formed, .and separated from a supernatant alcoholiclayer. To the separated aqueous phase approximately three times its volume of alcohol was added and salt was precipitated which after washing and drying was substantially pure ammonium thiosulphate. The

- said upper phase was treated with an equal volume of alcohol. Impure ammonium. thiosulphate was precipitated by the addition step, was filtered out, and returned to the original solution for" reprocessing. Approximately 82% of the contaminating thiosulphate oi. the said upper phase was removed by this second alcohol addi- --,tion. The filtrate comprised a solution oi ammonium thiocyanate contaminated with about 4.5% by weight of ammonium thiosulphate. The said filtrate was distilled at reduced pressure leaving a distillation residue comprising, at room temperature, an aqueous slurry of ammonium thiocyanate crystals. Such crystals, directly recovered, and without further purification, as by washing, were white, without'mercaptan odor,

and contained about 1.4% ammonium thiosulphate.

The alcohol referred to in the following claims is generally to be ethyl alcohol because of its obviously greater practicality.

The invention as hereinabove set forth is em-- bodied in particular form and manner but may be variously embodied within the scope of the claims hereinafter made.

I claim: a

1. A process for the separation and recovery of ammonium thiosulphate and ammonium thioing the steps of: separating the solution, by con-' trolled addition of alcohol thereto, into two liquid phases only, in one of which the thiocyanatethlosulphate ratio, and in the other the thiosulphate-thiocyanate ratio, is greatly increased over that ratio, respectively, existing in the original salt solution; separating the two liquid phases and treating each of the separated phases with alcohol to precipitate ammonium thiosulphate therefrom; thereafter washing the precipitate from the predominantly thiosulphate phase with alcohol thus providing substantially pure ammonium thiosulphate and returning precipitate from the other phase to the first said step; and evaporating, and crystallizing ammonium thiocyanate from the filtrate of the said other phase.

3. A process for the separation and recovery in substantially pure form, of ammonium thiocyanate and ammonium thiosulphate from solution-m'ixtures thereof, comprising the steps of separating the solution by controlled addition of alcohol thereto, into two liquid phases only, in

one of which the thiocyanate-thiosulphate ratio, and in the other the thiosulphate-thiocyanate ratio, is greatly increased over that ratio, respectively, existing in the original salt solution; separating the two liquidphases and treating such of the separated phases with alcohol to precipitate 'thiosulphate therefrom; thereafter washing the precipitate from the predominantly thiosulphate phase with alcohol thus providing substantially pure ammonium thiosulphate and returning the precipitate from the said other phase to the first said step; evaporating alcohol from the filtrates of the said phases; and crystallizing from the concentrated filtrates ammonium thiocyanate. 4

4. A process "for the separation and recovery, in substantially pure form, of ammonium thiocyanate and ammonium thiosulphate of aqueous such gas liquor to obtain a concentrated solution of the said salts; flowing alcohol into the said solution until a separate, liquid phase settles out thereby forming a two-phase system of two liquids only, a supernatant predominantly alcoholic layer containing a, major portion of the thiocyanate and a lower predominantly aqueous layer containing a major portion of the thiosulphate; separating the two liquid. phases and treating each phase out of contact with the other with a further quantity of alcohol to precipitate the thiosulphate content of each said phase; washing 'the-so-precipitated salt from the predominantly aqueous phase with additional quantity of alcohol to provide substantially pure ammonium thiosulphate; flowing the alcohol of the said washing steps into further quantities of concentrated solution; returning precipitated salt from the predominantly alcoholic phase to the first said step; distilling the said alcoholic phase for recoveryof its alcohol content, thereby providing a distillate of alcohol which can be returned in the cycle of the process and an aqueous distillation residue which contains ammonium thiocyanate; filtering and evaporating-the said residue; and crystallizing from the concentrated filtrate substantially pure ammonium thiocyanate.

5. A process for the continuous separation and recovery of ammonium thiocyanate and ammonium thiosulphate from an aqueous solution-mixture of the salts, comprising: flowing at a-measured rate such aqueous solution and alcohol into admixture in such relative proportion that a supernatant predominantly alcoholic phase and a lower, predominantly aqueous phase only are formed by said mixing; continuously separating, by decantation the two said phases; continuously flowing alcohol into admixture with each of the separated phases to precipitate from each ammo- 40 nium thiosulphate; continuously separating the from the so-formed evaporation residue.

6. A process for the continuous separation of ammonium thiosulphate and ammonium thiocyanate in aqueous solution-mixture thereof, and for the continuous recovery of substantially pure ammonium thiosulphate and the production of substantially pure ammonium thiocyanate, the

said process comprising the steps of continuous- ,ly flowing alcohol into the said aqueous solution until a separate, liquid phase settles out thereby forming a two phase system of two liquids only, in the upper one of which the thiocyanate-thlosulphate ratio, and in the lower the thiosulphatethiocyanate ratio is greatly increased over that ratio respectively existing in the original salt solution; continuously separating, by decantation,

the two phases; continuously flowing alcohol into admixture with each of the separated phases to precipitate ammonium thiosulphate from each phase; continuously separating the precipitate from the said aqueous phase, washing with alcohol and drying to provide substantially pure ammoniumthiosulphate; continuously flowing the alcohol of the said washing step into further quantities of aqueous solution; continuously returning, in aqueous solution, precipitate from the said alcoholic phase to untreated solution-mixsae. limmra comprisin the stem 01: eVlDOl-SW we; distilling the filtrate or t alcoholic pm assaces 5 for recovery of its alcohol content,'to provide a distillate of alcohol which is continuously returned to the first stage of the cyclic process and an aqueous distillation residue which contains ammonium thiocyanate; filtering and evaporating the residue; and crystallizing from the concentrated filtrate substantially pure ammonium thiocyanate.

7. In a process for the separation and recovery of ammonium thiocyanate and ammonium thiosulphate from aqueous solution-mixture thereof, the steps comprising: gradually adding alcohol to'such aqueous solution to form two liquid phases only, one predominantly aqueous and the other alcoholic; separating the phases; and adding alcohol to each of the separated phases to precipitate ammonium thiosulphate and washing the precipitate obtained from the aqueous phase to recover substantially pure ammonium thiosul- Dhate.

8. In a process for-the separation and recovery of ammonium thiosulphate and ammonium thiocyanate from an aqueous solution-mixture thereof the steps comprising: continuously flowing at a predeterminedrate such aqueous solution and alcohol into admixture in such proportion that a supernatant, predominantly alcoholic phase and a lower predominantly aqueous phase'only are formed by said mixing; continuously separating by decantation the said phases; continuously flowing alcohol into each 01 the separated phases to precipitate from each ammonium thiosulphate; and washing the precipitate obtained from the aqueous phase to recover substantially pure ammonium thiosulphate.

9. In a process for the separation and recovery of substantially pure ammonium thiosulphate and ammonium thiocyanate from aqueous solution-mixture containing a relatively high concentration of the ammonium thiocyanate salt, the steps comprising: evaporating the aqueous solution to a specific gravity of at most less than about 1.25; flowing at a predetermined rate such aqueous solution and alcohol into admixture in such proportion that a supernatant, predominantly alcoholic phase, and slower predominantly aqueous phase only are formed by said mixing; separating by decantation the said phases;

flowing alcohol into each of the separated phases to precipitate from each ammonium thiosulphate; andwashing the precipitate obtained from the aqueous phase to recover substantially pure ammonium thiosulphate.

- JOSEPH A. SHAW. 

